This invention relates to high temperature furnace muffles and more particularly to a new high temperature muffle that addresses problems with prior art designs and extends the useful life of the muffle.
It is well known to manufacture a wide variety of products and materials in furnaces, and the product or material proceeds through an elongated, hollow, controlled atmosphere furnace muffle. The muffle is required to operate in a hostile temperature environment and must cooperate with the furnace chamber, product work package (product or material), an existing furnace support structure, be adaptable to different process temperatures and atmospheres, and must accommodate different material and ingredient temperature-dependent characteristics. Some muffle design considerations include the cross-sectional shape, corrugated design for rigidity and strength, wall thickness, joint assembly, weld joint design, and forming method, although this list should not be deemed exhaustive of all design considerations. In these various application-specific environments, different configurations of the muffle are designed to meet or exceed desired performance requirements, extend component life, and likewise minimize operating and maintenance costs.
A common muffle design includes a convex, catenary arch along an upper surface of the muffle. The catenary arch cross-sectional shape is desirable because the arch equalizes stress throughout the section and thus provides improved strength and component life. The muffle includes a generally planar, horizontal surface that typically forms a first, lower surface of the muffle. Material or product enters into an inlet at a proximal or first end of the muffle and is advanced along the lower surface typically in a longitudinal direction toward an outlet located at a distal or second end of the muffle. Extending upwardly in substantially parallel relation from opposite edges of the lower surface are support surfaces, sidewalls, or reinforcing piers. Upper ends of the sidewalls receive terminal edges of the convex, catenary arch so that the lower surface, sidewalls, and the upper catenary arch form and enclose a muffle cavity that extends longitudinally from the inlet to the outlet. Belts, conveyors, pushing mechanisms, etc., advance the material or parts that enter the inlet at the first end of the muffle toward the outlet located at the second end of the muffle.
It is known to form the muffle of a durable material such as stainless steel and particularly a commonly used material is stainless steel nickel alloy, although other materials may be incorporated to address the need, for example, of different characteristics such as high temperature strength, carburization resistance, oxidation resistance, resistance to pitting, improved ductility, enhanced weldability, flaking prevention of an oxide surface, etc.
One failure mode of the muffle is buckling or collapse of the convex, catenary arch. Such a failure alters the cross-sectional profile of the material/product and gas flow through the muffle cavity. It may also result in a tear or a leak path resulting in a leak to atmosphere. Failure typically requires repair or replacement. It is believed that, although the configuration of the catenary arch design equalizes stress throughout the section which improves strength and component life, the convex arch (like the remainder of the muffle) is subject to thermal stresses from heating and cooling, as well as the harsh environment of the atmosphere within the muffle as it interacts with the materials and products transferred therethrough. A need exists to explore alternative configurations that may improve useful life and further delay potential failure associated with buckling or collapse of the convex catenary arch.
A need exists for an improved arrangement that enhances useful life, reduces use of atmosphere in the muffle, provides at least one or more of the above-described features and advantages, as well as still other features and benefits.